Alkali metal silicates, and more particularly sodium silicates, are commonly used in detergents compositions such as washing and dishwashing compositions.
Dishwashing powders generally consist of dry mixed granular components having a particle size between 200 and 2000 micrometers, preferably between 300 and 1000 micrometers. Although most fabric washing powders were produced by spray-drying a liquid mixture (slurry), including alkali metal silicate in liquid form, developments towards phosphate-free- and higher density powders have increasingly led to a change in processing to produce these fabric washing detergent powders, i.e.: Either towards a complete dry-mixing of all components in granular form or by spray drying only part of the components followed by post-dosing of the remaining components in granular form. The alkali metal silicate is preferably post-dosed in granular form in this process as well.
Alkali metal silicates used in detergents are preferably sodiumsilicates having molar ratios SiO.sub.2 /Na.sub.2 O varying between 1.0 (Sodium metasilicate used in machine dishwashing products) and 3.4. Generally preferred molar ratios (SiO.sub.2 /Na.sub.2 O) for modern machine dish washing and fabric washing powders being between 2.0 and 2.4.
Many formulations for dishwashing as well as for fabric washing contain bleach components which are based on peroxide. For detergent powder formulations, alkalimetal salts of perborate and percarbonate are generally preferred either alone or in combination with bleach promoters (e.g.: TAED, i.e.: tetraacetylethylenediamine).
In washing compositions, it is known, for example from WO92/03525, that heavy metals present can catalytically decompose peroxide bleach systems in the presence of water, e.g.: during the washing process. The heavy metal ions are either present in the washgoods (dirt/stains) or in the raw materials used for the detergent powder. It is also known that alkali metal silicates can bind heavy metals and have thus a positive effect on bleach stabilisation. However, during each washing cycle, part of said alkali metal silicates which have absorbed heavy metals, precipitates on the textile fibres, leading to a deposit on the cloth, which increases during each subsequent wash cycle. This heavy metal silicate gives rise to local decomposition of the bleach and to textile strength loss as well as to dye damage. This phenomenon has been for example disclosed in WO92/03525 pages 38-39. It is known that antiredeposition agents, such as certain polymers or copolymers, can in some circumstances prevent deposition of metal silicates.
Nevertheless, WO92/03525 has shown that adding such polymers has little effect on bleach stability and textile strength in presence of silicates (comparative example E).
Thus there is a need for a soluble alkali metal silicate in a granular form which can be added to bleach containing detergent compositions for its heavy metals sequestering capacity and which does not cause deposition of silicates onto the fabrics.
Turning to dishwashing compositions silicate compounds inhibit corrosion on glass while acting as alkali generating agents. Now, while being very active in such compositions, said compounds, specifically metasilicates, can be particularly dangerous when ingested by young children. Less alkalinity with a comparable level of protection against corrosion can be achieved by using sodium silicates with higher SiO.sub.2 :Na.sub.2 O molar ratios, e.g. disilicates (SiO.sub.2 /Na.sub.2 O molar ratio: 2.0). Nevertheless such products are still hazardous and more and more national legislations are requiring to further limit the alkalinity of the mechanical dishwashing compositions, something which is normally achieved by decreasing the volume of alkali metal silicate in the composition since the alternative of using alkali metal silicates with a molar ratio of more than 2.4 is not practical due to the poor dissolution properties of such a product.
In fabric washing powders, soluble alkali metal silicate with a molar ratio (SiO.sub.2 /Na.sub.2 O) of 2.0 is generally used and although the quantity of silicate used in fabric washing powders is generally much lower (e.g.: 2-7%) alkalinity is also becoming an issue in these products, also containing other alkaline components.
There is therefore a need for a soluble alkali metal silicate in a granular form which can be added in increasing amounts to detergent compositions without increasing the pH.